The present invention relates to a solid state image pick-up device with a shutter function comprising a number of charge coupled devices (hereinafter abbreviated as CCD) having a self-scanning function and a photoelectric converting function.
Generally, since the CCD can convert light into an electrical signal charge and can store and transfer the thus converted charge, it is widely utilized as the image pick-up element of the self-scanning type. In the case of using the CCD as the image pick-up element, there has been proposed to give the CCD image pick-up element itself a shutter function by controlling the charge storing period in the light receiving portion effecting the photoelectric converting operation from an external control unit. In this method, a charge transfer portion and an overflow drain portion are arranged adjacent to the light receiving portion through gate electrodes, respectively, and the shutter function is performed in such a manner that the charge stored in the period corresponding to a shutter speed (shutter open period) is transferred to the charge transfer portion and the charge generated in the other period is discharged to the overflow drain portion. The overflow drain (hereinafter abbreviated as OFD) region may be classified into the following two types, i.e. a lateral type in which the OFD region is arranged adjacent to the light receiving portion laterally and a vertical type in which the OFD region is arranged in a depth direction of the substrate. In the vertical OFD type, the stored charge is discharged into the substrate and thus it is possible to attain high sensitivity and high integration as compared with the lateral type.
However, in case of applying the shutter function to the CCD image sensor of the vertical OFD type, disadvantages mentioned hereinafter result.
At first, a typical construction of the CCD image sensor of vertical OFD type will be explained with reference to FIG. 1. In FIG. 1, a substrate 1 is formed by a common n-type semiconductor substrate and a p-type well 2 is formed by an ion implantation of, for example, boron into th n-type substrate 1, a depth of which is made thin partially in the vertical direction of the substrate 1. A light receiving portion 3 is formed on this thin portio of the well 2 and a vertically transferring portion 4 and a transferring gate 5 are arranged side by side on the remaining thick portion of the p-type well 2. The light receiving portion 3 comprises of a photodiode formed by a junction between the p-type well 2 and an n.sup.+ surface region 6 formed by diffusion in the surface of the thin portion of the p-type well 2. Moreover, the transferring portion 4 is induced in the p-type well 2 by means of a plurality of shift electrodes 8 which are provided above the well 2 through an insulating layer 7 having a predetermined thickness. The shift electrodes 8 are aligned in the vertical direction, i.e. a direction perpendicular to the plane of the drawings. The transferring gate 5 for controlling the transfer of the charge from the light receiving portion 3 to the transferring portion 4 is induced in a portion of the p-type well 2 between the light receiving portion 3 and the transferring portion 4 by means of a transferring gate electrode 9 provided above the well 2 via the insulating layer 7. Further, a channel stopper 10 is arranged to prevent a leakage of the stored charge from the relevant light receiving portion 3 to adjacent transferring portions 4 and is usually formed by a p.sup.+ diffusion region. Furthermore, a light shielding layer 11 is provided to shield all the surfaces other than the light receiving portion 3 from incident light.
Next, an operation of the known image pick-up device having the construction mentioned above will be explained. At first, it is assumed that a reverse bias voltage V.sub.s is applied between the p-type well 2 and n-type substrate 1 as shown in FIG. 1, and the p-type well 2 is in a depletion condition. Then, a distribution of an electron potential .phi. viewed from the light receiving portion 3 to the substrate 1 is shown by the curve a in FIG. 2. Under such a condition, when light 12 is made incident upon the light receiving portion 3, electron-hole pairs are generated in the n.sup.+ region 6, with the amount of the pairs corresponding to the incident light amount. Then, the thus generated electrons are stored in the n.sup.+ region 6 due to an electric field slope and the thus generated holes are discharged to the earth potential through the p-type well 2. In this case, the potential of the n.sup.+ region 6 is decreased corresponding to an increase of the stored electrons, and finally the potential thereof has a distribution illustrated by the curve b in FIG. 2. At the same time, the potential of the p-type well 2 becomes small. When a potential difference between the n.sup.+ region 6 and the p-type well 2 is substantially equal to a diffusion potential .phi..sub.b, the charge stored in the n.sup.+ region 6 flows into then-type substrate 1, because the forward bias is applied across the n.sup.+ region 6 and the p-type well 2. In this manner, the n-type substrate 1 serves as the overflow drain for discharging the charge out of the light receiving portion 3. In this case, the device must be so constructed that the stored charge does not flow into the transferring portion 4. This may be done by, for example, making an acceptor concentration in a portion of the p-type well situating just below the transferring portion 4 high than that of the light receiving portion 3.
In order to perform the shutter function in the CCD image sensor mentioned above, at first, two clock pulses .phi..sub.1 and .phi..sub.2 as shown in FIGS. 3A and 3B are alternately applied to each of a plurality of shift electrodes 8. A predetermined pulse voltage V.sub.t having a pulse width T.sub.2 shown in FIG. 3C is applied to the transferring gate electrode 9 at a given interval (for example, one field scanning period T.sub.1). The pulse width T.sub.2 denotes a period during which the signal charge stored in the light receiving portion 3 is transferred to the transferring portion 4 through the opened transferring gate 5. Moreover, a voltage V.sub.s.sbsb.2 which is higher than a standard voltage V.sub.s.sbsb.1 is applied to the n-type substrate 1 at a predetermined interval (for example, one field scanning period T.sub.1). In a period T.sub.3 during which the voltage V.sub.s.sbsb.2 is applied, the potential distribution is shown by a curve c in FIG. 2. In this case, the potential barrier between the n.sup.+ diffusion region 6 and the p-type well 2 becomes smaller than the diffusion potential and thus a part of the stored charge flows into the n-type substrate 1. In a period T.sub.4 during which the standard voltage V.sub.s.sbsb.1 is applied to the substrate 1, the signal charge is always stored. Therefore, it is possible to adjust the charge storing period T.sub.4, i.e. the shutter speed to an arbitrary value within the one field scanning period T.sub.1 by changing from the external a ratio between the periods T.sub.4 and T.sub.3 of the bias voltage across the substrate 1 and the well 2 as shown in FIG. 3D. Further, the charge stored in the shuter opening period T.sub.4 is transferred to the transferring portion 4 during the transferring gate opening period T.sub.2 and further transferred to a vertically transferring portion (not shown) by means of the clock pulses .phi..sub.1 and .phi..sub.2.
However, in the CCD image sensor of the vertical OFD type mentioned above, since the discharging operation of the signal charge from the n.sup.+ region 6 to the n substrate 1 during the period T.sub.3 is performed through the potential barrier between the n.sup.+ region 6 and the p-type well 2, a substantial amount of the signal charge is remained in the n.sup.+ region 6 even if a sufficiently high reverse bias voltage is applied to the n-type substrate 1. In this case, if the signal charge remaining is large, the sensitivity of the sensor is decreased correspondingly, and also if the amounts remaining of each pixels consisting of the image pick-up element are different from each other, the sensitivities of each pixels vary correspondingly from pixel to pixel.